Transducers generally convert electrical signals to mechanical signals or vibrations, and/or mechanical signals or vibrations to electrical signals. Acoustic transducers in particular convert electrical signals to acoustic signals (sound waves) in a transmit mode (e.g., a speaker application), and/or convert received acoustic waves to electrical signals in a receive mode (e.g., a microphone application). The functional relationship between the electrical and acoustic signals of an acoustic transducer depends, in part, on the transducer's operating parameters, such as natural or resonant frequency, acoustic receive sensitivity, acoustic transmit output power and the like.
Transducers, such as ultrasonic transducers, are provided in a wide variety of electronic applications, including filters. As the need to reduce the size of many components continues, the demand for reduced-size transducers continues to increase, as well. This has lead to comparatively small transducers, which may be micromachined according to various technologies, such as micro-electromechanical systems (MEMS) technology.
Various types of MEMS transducers, such as piezoelectric ultrasonic transducers (PMUTs), include a resonator stack, having a layer of piezoelectric material between two conductive plates (electrodes), formed on a thin membrane. To provide stable and predictable operation, the membrane is typically designed to have a net tensile stress. This avoids issues arising from a buckled membrane. However, the resonant frequency of a tensile membrane is very sensitive to in-plane stress. Further, the film stress typically has poor control compared to control achieved for film thickness.